1. Field of the Invention
The present invention relates to the encapsulation of at least one electronic component by use of a molding composition comprising a thermosetting prepolymer. More especially, the present invention relates to an encapsulation process featuring low-pressure transfer and injection molding techniques.
By the term "encapsulation" as utilized herein, there is intended a process in which the electronic component is:
(i) either totally covered or enveloped within the molding composition, which thus serves as an envelope or complete casing;
(ii) or, if the component is in direct contact with a given support over one or more of its face or other surfaces, partially covered with the molding composition, which thus serves as a cover or coating for those surfaces which are not in intimate contact with the support.
2. Description of the Prior Art
The process of encapsulation by transfer is a well-known and very widespread coating method because it makes it possible to achieve high productivity and, consequently, to reduce production costs. Such process consists in placing the components to be encapsulated within the cavities of a heated mold, and in introducing or charging the molding composition into the cavities, completely filling same while enveloping said components, with the aid of a piston filling means. In order not to damage the electronic components, it is typically recommended to carry out the molding operation at a temperature which is generally between 130.degree. and 200.degree. C., and under a pressure which is generally below 80 kg/cm.sup.2. For reasons of productivity, it is advantageous for the molding cycle to be completed within a short period of time of not more than 3 minutes.
Injection molding is a similar method, also well-known to this art, in which the molding composition is converted to the molten state and injected directly into the mold cavities by means of a screw extruder.
The aforenoted molding techniques require the use of a molding composition which, under the conditions of temperature prevailing during the molding operation, must possess a viscosity and a gellng time which have well-established values. In fact, a high viscosity can damage the electronic components, while an excessively low viscosity can be the cause of objectionable porosity.
The gelling time must correspond very precisely to values which make it possible to easily carry out each molding cycle within the short period mentioned above. An excessively short gelling time can lead to incomplete filling of the mold cavities, while excessively long gelling can allow the molding composition to flow or "flash" between the planar interface of the mold halves. These burring or flashing effects mandate additional cleaning operations, which are expensive and difficult to perform. A good balance between the parameters of viscosity and gelling time is thus essential for economic production.
The molding composition must also possess other characteristics. These relate to the following characteristics:
(i) non-flammability and non-toxicity;
(ii) chemical purity;
(iii) high heat resistance combined with good electrical properties and excellent dimensional stability;
(iv) good mechanical strength; and
(v) good adhesion to a variety of substrates, in particular to metal substrates (for example, the metal baseplates with which the electronic components may be in contact, and the connecting wires).
The molding compositions which are currently marketed are essentially epoxide resins and silicone resins. However, although their characteristics in terms of viscosity and gelling time can be well adapted to suit the molding conditions indicated above, in certain respects these resins are inadequate and do not totally satisfy the manufacturers of electronic components, in particular the manufacturers of active power components, such as, for example, diodes, transistors and integrated circuits, as well as the manufacturers of passive components, such as, for example, windings, rectifiers, ferrite antennae, condensers, resistors and connectors. An important problem which should normally be solved in the field of the encapsulation of power components, which, as is known, are materials which are typically fairly sensitive to atmospheric pollution, arises from the need to utilize a molding composition which possesses very good properties in terms of heat resistance and mechanical flexural strength; in fact, these properties govern both the leaktightness of the casings or covers and their adhesion to metal substrates.
Epoxide resins, which have high coefficients of expansion, are at the limit of their heat resistance capabilities, and silicones lack mechanical flexural properties, crack and adhere poorly to metal substrates.